This disclosure relates to a method of making an antistatic agent.
Thermoplastics are useful in the manufacture of articles and components for a wide range of applications, from automotive parts to electronic appliances. Because of their broad use, particularly in electronic applications, it is desirable to provide thermoplastic resins with antistatic agents. Many polymers or blends of polymers are relatively non-conductive, which can lead to static charge build-up during processing and use of the polymer. Charged molded parts, for example, may attract small dust particles, and may thus interfere with a smooth surface appearance, for example by causing a decrease in the transparency of the article. In addition, the electrostatic charge may be a serious obstacle in the production process of such polymers.
Anti-static agents are materials that are added to polymers to reduce their tendency to acquire an electrostatic charge, or, when a charge is present, to promote the dissipation of such a charge. Organic anti-static agents are usually hydrophilic or ionic in nature. When present on the surface of polymeric materials, they facilitate the transfer of electrons and thus eliminate the build up of a static charge. Anti-static agents have also been added to the polymer composition before further processing into articles, and may thus be referred to as “internally applied.” Useful anti-static agents applied in this manner are thermally stable and able to migrate to the surface during processing.
A large number of anti-static agents having surfactants as their main constituent have been considered and tried. Many suffer from one or more drawbacks, such as lack of compatibility with the polymer (which interferes with uniform dispersibility), poor heat stability, and/or poor antistatic characteristics. Poor heat resistance in particular can adversely affect the optical properties of engineering thermoplastics such as aromatic polycarbonates.
Particular phosphonium salts of certain sulfonic acids, however, have been shown to be useful antistatic agents. U.S. Pat. No. 4,943,380 discloses reducing the static charge on polycarbonate resins with an anti-static composition containing 90–99.9 weight % of polycarbonate and 0.1–10 weight % of a heat resistant phosphonium sulfonate having the general formula:
wherein R is a straight or branched chain alkyl group having 1 to 18 carbon atoms; R1, R2 and R3 are the same, each being an aliphatic hydrocarbon having 1 to 8 carbon atoms or an aromatic hydrocarbon group having 6 to 12 carbon atoms; and R4 is a hydrocarbon group having 1 to 18 carbon atoms.
U.S. Pat. No. 6,194,497 discloses antistatic resin compositions, particularly transparent resin compositions, comprising a thermoplastic polymer and a halogenated medium- or short-chain alkylsulfonic acid salt of a tetrasubstituted phosphonium cation. The antistatic agent described therein is prepared by ion exchange of a potassium haloalkylsulfonate to produce the corresponding acid. The haloalkylsulfonic acid is then reacted with tetrabutylphosphonium hydroxide to product the antistatic agent.
An advantage of this synthesis is that use of an ion exchange step during synthesis results in a product that is very pure, i.e., contains little to no halogenated compounds that may ultimately lead to degradation of resins such as polycarbonates. However, while suitable for its intended purposes, this particular synthesis also has a number of drawbacks. For example, use of an ion exchange step increases the expense of the process, and may lead to the production of waste requiring disposal procedures. The synthesis also uses the potassium salt as a starting product, which is prepared from the corresponding sulfonylfluoride. Since the solubility of potassium peralkylsulfonates is relatively low, e.g., on the order of 5% at 20° C., a water/ethanol mixture is needed in the ion exchange. The flammability of ethanol requires the implementation of significant safety precautions during the synthesis. In addition, selecting the appropriate water/ethanol ratio is also important. An excess of alcohol may render the final product soluble in the reaction solvent, such that isolation of the product may require a further extraction step.
There accordingly remains a demand in the art for more efficient processes, particularly one-step processes, for making phosphonium sulfonate antistatic agents, as well as thermoplastic resin compositions that incorporate these antistatic agents. It would further be desirable for such processes to produce the antistatic agent in good yields without having a detrimental effect on the safety of the process and/or the purity of the product.